The forward converter is a common circuit topology used to transform electric energy from a source at a given potential to a destination load at a different potential. Typically, the forward converter requires fewer components than other converter topologies and is smaller and lighter. The standard forward converter comprises a transformer having a primary winding, a secondary winding, and a third winding. The primary winding is coupled to a source of power, usually DC power, via a primary switch and the secondary winding is coupled to a load via two commutating diodes. The primary switch generally comprises a semiconductor switching device such as a field-effect transistor (FET) or a bipolar-junction transistor (BJT). When the primary winding is energized by the closing of the primary switch, energy is immediately transferred to the secondary winding, hence the name forward converter. The third winding is coupled to the power source via a rectifying diode and serves to reset the ferromagnetic core of the transformer when the primary switch is opened. The transformer's third winding provides a current path for discharging the transformer's magnetizing current, which is created when the primary winding is energized.
The standard forward converter is particularly well suited for low and medium power-conversion applications. However, it is not as efficient as other topologies in converting power, particularly in low power applications. The following factors contribute to the lower efficiency of the forward converter.
First, the core-reset operation in the forward converter using the third winding is not as efficient as other core-reset operations in other converter topologies. The rectifier in series with the third winding causes an amount of power dissipation and, more importantly, the addition of a third winding to the transformer increases the transformer's dimensions and thereby reduces the transformer efficiency by increasing the core losses, the resistance of each winding, and the self inductance of each winding.
Second, the forward converter only uses the first quadrant of the transformer's core B-H characteristic, leaving the third quadrant unused. As a result, the size of the transformer must be increased to enlarge the operating range of the first quadrant at the expense of higher core losses and higher winding resistances.
Third, the power dissipation in the primary switch when it is switched on (a turn-on event) is greater than the power dissipation in comparable switches in other topologies. The power dissipation in a switch during a switching event depends upon the product of the voltage across the switch and the current through the switch. In a forward converter, the voltage difference across the primary switch changes from a value equal to the input voltage of the power source to a value near zero when the switch is closed. Additionally, the primary current begins immediately since the forward converter provides current to the secondary winding immediately upon energizing the primary winding. The high input voltage and the instantaneous current flow in the primary switch leads to a high power dissipation loss in the primary switch. The power dissipation losses during switching events become more significant as the switching frequency of the forward converter is increased, as is done to improve the conversion efficiency of the converter's transformer. The direct power dissipation losses become more significant as the switching frequency increases because the duration of each switching event comprises a larger fraction of each switching cycle duration as the switching frequency increases.
The above first and second factors have been addressed in U.S. Pat. No. 4,441,146 issued to Vinciarelli. In U.S. Pat. No. 4,441,146, the third winding is eliminated and replaced by a series combination of a storage capacitor and an auxiliary switch coupled across either the primary or secondary winding. The auxiliary switch is operated counter to the primary switch, i.e., the auxiliary switch is open when the primary switch is closed and closed when the primary switch is open. When the primary switch is open, the storage capacitor and auxiliary switch operate to capture and store the transformer's magnetizing current, which was built up when the primary winding was energized, and then to return the magnetizing current to the transformer in a manner which resets the transformer's core. The elimination of the third winding addresses the first above efficiency factor. Additionally, the returning of the magnetizing current to the transformer creates a condition where the first and third quadrants of the transformer core's B-H characteristic are utilized, thereby addressing the second above efficiency factor.
The third efficiency factor, however, is not addressed by the prior art. In this respect, the present invention provides improvements to the basic forward-converter topology by reducing the power dissipation during switching events, thereby increasing the power-conversion efficiency of the basic forward-converter topology.